Electromagnetic waves are widely used in areas ranging from communication and radiolocation to TV broadcasting, imaging, medical treatment, and food processing. Electromagnetic waves in the microwave, millimeter-wave, and in sub-millimeter wave ranges are particularly useful for the purposes of communication due to relatively high carrier frequency and associated ease of beaming the wave and comparatively large information carrying capacity. Electromagnetic waves in the sub-millimeter wavelength region, or so-called terahertz radiation, are presently used in non-invasive through-imaging applications.
Devices for collimating or focusing electromagnetic waves are important elements of many types of antenna devices. Quite often, the term “antenna” is used in the prior art to denote the collimating or focusing elements themselves. Antennas vary widely in their construction. One of the most traditional and well-known antennas is a reflective antenna, such as paraboloid reflector antennas for a satellite TV reception. Transmission antennas, such as hyperboloid dielectric lenses, are also known, although they are not as widely used due to their larger mass as compared to reflective antennas. Furthermore, holographic principles have been applied at microwave frequencies for designing low-profile scattering surfaces for high-gain reflective and transmissive antenna applications. A review of microwave holography can be found in an article by W. E. Kock entitled “Microwave Holography”, Microwaves, vol. 7, no. 11, pp. 46-54, November 1968, which is incorporated herein by reference.
Reflective antennas can be constructed in form of concave reflectors, Fresnel zone plates, or in form of so called “artificial impedance” reflectors. Fresnel zone plates and artificial impedance reflectors are thinner than concave reflectors, however they generally suffer from a lower aperture efficiency, as well as somewhat limited bandwidth. A considerable effort has been devoted to developing a so-called “reflectarray” technology, where the curved reflector surfaces are replaced by thin flat panels of microstrip patches. For example, in U.S. Pat. No. 4,684,952 by Munson et al., which is incorporated herein by reference, a microstrip reflectarray for satellite communications is disclosed. The major disadvantage of reflectarrays is their limited bandwidth. Furthermore, due to the difficulty in reproducing the required interference patterns at microwave frequencies, holographic antennas, artificial impedance antennas, and reflectarrays generally suffer from lower aperture efficiencies than conventional reflectors or dielectric lenses.
Each technology has its strengths and weaknesses, and the requirements of the application will usually dictate the type of antenna to be selected. Conventional paraboloid reflectors and dielectric lenses in general have a higher radiation efficiency, but they require a larger volume than planar arrays. Planar arrays are attractive for their low profile and capability for electronic beam scanning, but these advantages come at the expense of complex feed network design and reduced radiation efficiency. For fixed-beam applications, conventional reflectors and lenses usually have superior electrical performance and would probably always have been selected if it were not for the larger volumes they occupy.
Regardless of technology used, transmissive antennas offer certain advantages over reflective ones, such as the elimination of aperture blockage by the feed antenna and reduced sensitivity to manufacturing tolerances, both of which are important for higher frequency designs. However, less work has been carried out on reducing the volume of transmissive antennas. In most cases, transmissive antennas are lenses formed out of dielectric material with a plano-hyperbolic cross-section. These lenses are relatively thick, especially for designs with small focal length-to-diameter (F/D) ratios. The lenses can be zoned to reduce the overall thickness, but the zoning results in reduced bandwidth and aperture efficiency of the lens.
Referring to FIG. 1A, a prior-art dielectric lens 100 is shown. The dielectric lens 100 is shaped to transform a planar wavefront 101 into a spherical wavefront 102 or vice versa. The dielectric lens 100 introduces a phase delay at its center 103 larger than a phase delay at its edge 104, whereby the spherical wavefront 102 is formed. Factors affecting the performance of the dielectric lens 100 include the impedance mismatch at the boundaries and the transmission loss in the material of the dielectric lens 100. As noted above, the dielectric lens 100 has a comparatively good performance, although it is too bulky and heavy for many applications.
Referring now to FIG. 1B, a prior-art Fresnel zone plate 105 is shown. The Fresnel zone plate 105 operates by blocking portions of the incoming wavefront 101 by metal structures 106 to obtain the spherical wavefront 102 due to diffraction. The Fresnel zone plate 105, although being much thinner and lighter than the dielectric lens 100, has a focusing efficiency of about 6 dB worse than a dielectric lens 100, because approximately half of the incoming electromagnetic energy is blocked by the metal structures 106 of the Fresnel zone plate 105.
An effort has been undertaken in the prior art to provide a transmissive antenna that would combine the compactness of the Fresnel zone plate 105 with the performance of the dielectric lens 100. Two approaches have been tried in the prior art. One approach is to use so called “artificial dielectric” as a material for the lens. The artificial dielectric is a composite material consisting of a dielectric host containing an array of metal inclusions, thus modifying an effective dielectric constant of the composite material. By spatially varying the density of the inclusions to make the effective refractive index of the lens higher at the lens center than at its edges, the desired focusing property of the lens can be achieved without having to make the lens as thick as the traditional dielectric lens 100.
Volume holographic elements can also be created using the artificial dielectric approach. For example, in U.S. Pat. No. 6,987,591 by Shaker et al., incorporated herein by reference, an artificial dielectric-type volume hologram is disclosed. Disadvantageously, the artificial dielectric approach, although reducing the antenna thickness, still results in a relatively thick, heavy, and expensive antenna device, because many layers of metal inclusions, typically 80 or more, are required for a satisfactory performance to be obtained.
Another prior-art approach to reduce thickness of a transmissive-type antenna is to use an array of microstrip patches. Turning to FIG. 1C, a microstrip device 107 is schematically shown in a side view. The microstrip device 107 includes an array of resonant patch receivers 108A, 108B, 108C, 108D, and 108E coupled to an array of microstrip delay lines 109A, 109B, 109C, 109D, and 109E coupled to an array of resonant patch transmitters 110A, 110B, 110C, 110D and 110E, respectively. The delay time of the microstrip delay lines 109A to 109E varies, increasing in going from the top microstrip delay line 109A to the center microstrip delay line 109C, and decreasing in going from the center microstrip delay line 109C to the bottom microstrip delay line 109E. The microstrip delay times are selected so that the planar wavefront 101 is transformed into the spherical wavefront 102 or vice-versa. One drawback of the microstrip device designs is that the resonant patch receivers 108A to 108E have to be spatially separated from corresponding resonant patch transmitters 110A to 110E by a gap 111 containing the microstrip delay lines 109A to 109E, which increases the mechanical complexity and thickness of the microstrip device 107. Furthermore, the separation between the elements on the same layer (108A to 108E; 110A to 110E) has to be about one wavelength, which results in a significant quantization error. Another drawback is a reduced bandwidth due to the resonant character of the patch receivers 108A to 108E and the patch transmitters 110A to 110E.
Yet another prior-art approach to create a low-profile transmissive antenna is to use so-called transmitarrays. Transmitarrays use a small number of thin dielectric layers to emulate a lens behaviour. A prototype transmitarray consisting of four dielectric sheets upon which thin cross dipoles were printed was demonstrated by M. R. Chaharmir et al. in an article entitled “Cylindrical Multilayer Transmitarray Antennas,” International URSI Commission B Electromagnetic Theory Symposium, EMTS-2007, Ottawa, Canada, July 2007, incorporated herein by reference. One drawback with current transmitarray designs is the requirement for an air gap between dielectric layers of one tenth of a wavelength or more, to maximize radiation efficiency. This increases the mechanical complexity of the device and does not allow for achieving an optimum thickness reduction. Nevertheless, a transmitarray is usually much thinner than the shaped dielectric lens 100.
Finally, it is important to mention research carried out on the use of holographic techniques for designing low-profile antennas and lenses at microwave frequencies, as disclosed in an article by K. Iizuka et al. entitled “Volume-Type Holographic Antenna,” IEEE Transactions on Antennas and Propagation, vol. 23, no. 6, pp. 807-810, November 1975; in an article by K. Lévis et al. entitled “Ka-band Dipole Holographic Antennas,” IEE Proceedings on Microwaves, Antennas and Propagation, vol. 148, no. 2, pp. 129-132, April 2001, and in an article by M. Elsherbiny et al. entitled “Holographic Antenna Concept, Analysis, and Parameters,” IEEE Transactions on Antennas and Propagation, Vol. 52, No. 3, pp. 830-839, March 2004, all of which are incorporated herein by reference. Disadvantageously, due to the difficulty in recording the phase pattern at microwave frequencies, these antennas were all of the amplitude type and consequently suffered from low aperture efficiencies, similar to Fresnel zone plate lenses disclosed by A. Petosa et al. in an article entitled “Comparison of an Elementary Hologram and Fresnel Zone Plate,” The Radio Science Bulletin, no. 324, pp. 29-36, March 2008, which is incorporated herein by reference.
An ideal electromagnetic lens device would work in transmission, have minimal reflection and transmission losses, operate over a wide bandwidth, have a thin flat profile, be lightweight and inexpensive to manufacture. The prior art is lacking a transmission antenna device that would have a relatively high efficiency, while being inexpensive, lightweight, and thin.
The present invention provides a transmissive phase element that is electrically thin, inexpensive, and lightweight, while being capable of introducing a predetermined arbitrary phase shift pattern into an electromagnetic wave for focusing, collimating, redirecting, or splitting the electromagnetic wave in almost arbitrary manner. This versatile performance is achieved without introducing an excessive loss in the path of the electromagnetic wave. Furthermore, a phase element of the present invention can also introduce a predetermined arbitrary amplitude shift pattern in addition to the phase shift pattern. The amplitude shifting property can be used, for example, for electromagnetic beam shaping and pattern synthesis.